1. Field of Invention
The invention relates to a dynamic radio frequency ripple signal compensator, and more particular to that applied in an optical storage system.
2. Related Art
The operation mode of the radial laser spot positioning system of an optical storage device falls into two types: the track following mode and the track seeking mode. The track seeking mode is further divided into long distance track seeking mode and short distance track seeking mode. During a short distance track seeking operation, the laser spot position and velocity relative to the tracks on an optical disk are detected by measuring the tracking error signal and the radio frequency ripple signal, whose phase difference is 90 degrees. Here, the radio frequency ripple signal is also named as focus sum signal or track sum signal. However, the intensity of the reflected laser from the optical disk is easily affected by a lens shift of the optical pickup head, fingerprints or dirt on the disk, or the reflectivity irregularity of the disk. This situation causes drop-out of the center level or distortion on the track cross waveform of the radio frequency ripple signal, which results in position or velocity detection error. Therefore, the stability of the radial laser spot positioning system of the optical storage device cannot be maintained at this moment.
U.S. Pat. No. 6,167,001 discloses two approaches to generate the radio frequency ripple signal. One approach uses a peak/bottom detector to generate the signal, and the other uses a low pass filter.
FIG. 1 illustrates the peak/bottom detector method to generate the radio frequency ripple signal. The photo detectors 101˜104 detect the laser beam reflected from the optical disk to the pickup head. The other two photo detectors 105 and 106 detect the tracking error signal of the CD disk, TECD. The output signals of the photo detector 105 and the photo detector 106 are delivered to the differential preamplifier 109, and then the high frequency noise in the signal is filtered by the first low pass filter 112. The tracking error signal TECD of the CD disk is then obtained. The output signals of the photo detector 101, the photo detector 102, the photo detector 103 and the photo detector 104 are delivered to the differential phase detection preamplifier 108, and the high frequency noise in the signal is then filtered by the second low pass filter 111. The tracking error signal of the DVD disk, TEDVD, is then obtained. The outputs signals of photo detectors 101˜104 are also delivered to the summing preamplifier 107 for generating the radial frequency signal, RF. The radial frequency signal is then delivered to the peak/bottom detector 110 to generate the radial frequency ripple signal, RFRP.
The peak/bottom detector 110 detects the peak/bottom value of the radio frequency signal after receiving the signal, and dynamically determines the central level of the radio frequency signal by averaging the peak/bottom value of the radio frequency signal. The central level of the signal is used to generate the radio frequency ripple signal. In the peak/bottom detector 110, the dynamic response of the peak detector is slower, while the dynamic response of the bottom detector is faster.
FIG. 2 illustrates the related signals of FIG. 1. The number 201 denotes the radio frequency signal RF. The number 202 denotes the radio frequency signal peak envelope RFPE, while 203 denotes the radio frequency signal bottom envelope RFBE. The tracking error signal TEDVD of the DVD disk is symbolized by the number 204, while the tracking error signal TECD of the CD disk is symbolized by 205. The radio frequency ripple signal RFRP uses the number 206. The optical disk 209 has lands 207 and grooves 208. It is obvious from the figure that there is a phase difference with 90 degrees between the tracking error signal (TEDVD or TECD) and the radio frequency ripple signal RFRP.
The greater noise and lower accuracy of the generated radio frequency ripple signal are the main drawbacks of this approach as shown in FIG. 2. Also, in order to eliminate the high frequency noise of the tracking error signal TECD of the CD disk or that of the tracking error signal TEDVD of the DVD disk, the tracking error signal (TEDVD or TECD) is delivered to a low pass filter (the first low pass filter 112 or the second low pass filter 111). This leads to a phase difference between the radio frequency ripple signal RFRP and the tracking error signal TECD of the CD disk or a difference between the radio frequency ripple signal RFRP and the tracking error signal TEDVD of the DVD disk. The phase difference varying with the frequency of the above signals destroys the 90-degree phase difference relationship between the radio frequency ripple signal and the tracking error signal. This situation may result in position or velocity detection error during a short distance track seeking operation.
Another approach for generating the radio frequency ripple signal is disclosed in FIG. 3. The radio frequency signal is generated by the summing preamplifier 107, which sums the output signals of the photo detectors 101˜104. The high frequency components of the radio frequency signal is then filtered by the third low pass filter 301, thereby obtaining the radio frequency ripple signal RFRP. This approach provides the same phase difference as the radio frequency ripple signal RFRP and the tracking error signal. The radio frequency ripple signal RFRP filtered by the third low pass filter 301 is cleaner. Therefore, the 90-degree phase difference relationship between the radio frequency ripple signal and the tracking error signal may be maintained. As shown in FIG. 4, there is less noise in the radio frequency ripple signal 401.
No matter which approach is adopted to generate the radio frequency ripple signal, the drop-out of the central level of the radio frequency ripple signal RFRP always appears due to the intensity variation of the laser beam reflected from the optical disk to the pickup head. There are two reasons for this phenomenon. The first is the lens shift of the optical pickup head. The intensity of the leaser beam reflected from the optical disk does not weaken. However, the lens shift causes the photo detectors to be unable to receive all of the reflected laser beam. The first phenomenon often happens in short distance track seeking operations, as illustrated in FIG. 5. As shown in FIG. 5, 501 represents the tracking error signal TEDVD of the DVD disk. 502 represents the radio frequency ripple signal RFRP. 503 is the radio frequency signal RF. 504 is the peak envelope of the radio frequency signal RFPE, while 505 is the bottom envelope of the radio frequency signal RFBE. Because of the lens shift during the short distance track seeking operation, the photo detectors cannot receive all of the reflected laser beam, and drop-out of the central level of the radio frequency ripple signal occurs.
The other reason for the drop-out phenomenon is due to the fingerprints or dirt on the disks or reflectivity irregularity of the disks, which causes variation of the intensity of the reflected laser beam from the disk to the pickup head. As illustrated in FIG. 6, 601 represents the tracking error signal TEDVD of the DVD disk. 602 represents the radio frequency ripple signal RFRP. 603 is the radio frequency signal RF. 604 is the peak envelope of the radio frequency signal RFPE, while 605 is the bottom envelope of the radio frequency signal RFBE. The radio frequency ripple signal 602 undergoes a serious drop-out at its central level, and the fingerprints on the disks also distort the track cross waveform. Comparing the two types of drop-out, the shift level of the first type drop-out is smaller and the dynamic response in frequency domain is low, while that of the second type is much more acute and the dynamic response in frequency domain covers a wide range, including the frequencies of the track cross waveform. Accordingly, the track cross waveform of the radio frequency ripple signal may be distorted.
U.S. Pat. No. 6,041,028 and No. 6,167,011 and U.S. laid-Open No. 20020141314 and 20020181374 disclose some solutions to solve the drop-out problem of the radio frequency ripple signal. These approaches convert the radio frequency ripple signal into a digitized RFRP zero cross signal, RFZC, first. Then, the symmetry error of the digitized RFRP zero cross signal, RFZC, is employed to compensate the radio frequency ripple signal. When the drop-out of the radio frequency ripple signal appears, the duty cycle of the digitized RFRP zero cross signal, RFZC, becomes asymmetric. And the symmetry error is then integrated by an integrator in the compensation loop and the output of the compensation loop is used to compensate the radio frequency ripple signal. However, the dynamic response of the compensation loop is also limited by the integrator. Therefore, the compensation for the drop-out of the central level of the radio frequency ripple signal caused by fingerprints or dirt on the disks which covers a wide range in frequency domain is not satisfactory.
U.S. laid-Open No. 20020093904 also discloses another approach to compensate the drop-out of the radio frequency ripple signal. This method involves delivering the radio frequency ripple signal to a peak detector and a bottom detector. Then, the average of the output signals of the peak detector and the bottom detector is employed as the central level of the radio frequency ripple signal. Although the disclosed approach solves the drop-out of the central level caused by lens shift, the drop-out caused by the fingerprints or dirt on the disk still cannot be compensated. This is because the dynamic response of the compensation loop cannot be faster than the frequencies of the track cross waveform, as it would need to detect the peak value and the bottom value of the track cross waveform of the radio frequency ripple signal in the peak detector and the bottom detector. Slower response results in ineffective detection of the drop-out of the central level and inability to compensate.